Thin film transistors (TFTs) are one of field effect transistors and include a semiconductor film with a thickness of approximately several to several hundreds of nanometers, which is formed over an insulating surface, as an active layer. The thin film transistors are widely applied to flat panel displays such as liquid crystal display devices and light-emitting devices and electronic devices such as integrated circuits (ICs).
In recent years, transistors including a metal oxide as an active layer as well as silicon or germanium have developed. The metal oxide is used for various applications. For example, indium oxide is a well-known metal oxide and used as a material of a transparent electrode included in a liquid crystal display device or the like. Some metal oxides have semiconductor characteristics and are called oxide semiconductors. The examples of such metal oxides having semiconductor characteristics are a tungsten oxide, a tin oxide, an indium oxide, a zinc oxide, and the like. A transistor in which a channel formation region is formed using such a metal oxide having semiconductor characteristics is known (Patent Documents 1 to 4 and Non-Patent Document 1).
As metal oxides, not only single-component oxides but also multi-component oxides are known. For example, InGaO3(ZnO)m (m is natural number) having a homologous phase is known as a multi-component oxide semiconductor including In, Ga, and Zn (Non-Patent Documents 2 to 4). Furthermore, it is confirmed that an oxide semiconductor including such an In—Ga—Zn-based oxide is applicable to a channel layer of a transistor (Patent Document 5 and Non-Patent Documents 5 and 6).